1. Field of the Invention
The present invention relates to a piezoelectric oscillator, for example, with crystalline quartz as a piezoelectric vibrator, and particularly relates to a Colpitts oscillator which oscillates stably by principal vibration and suppresses unwanted vibration.
2. Description of the Related Art
Piezoelectric oscillators are used for many electronic devices including a mobile communication base station because of their high stability of frequency. When an SC-cut quartz vibrator or an IT-cut quartz vibrator is used as a piezoelectric vibrator of the piezoelectric oscillator, the stress sensitivity characteristic, shock resistance characteristic, and so on are superior to those when an AT-cut quartz vibrator is used, so that the SC-cut quartz vibrator or the IT-cut quartz vibrator has been used a lot in recent years.
FIG. 7 is a diagram showing a resonance characteristic of the SC-cut vibrator. The horizontal axis represents the frequency, and the vertical axis represents the reactance. As is clear from this figure, in addition to a C mode (thickness shear vibration) as principal vibration, an A mode (thickness extensional vibration) and a B mode (thickness twist vibration) exist. In particular, the B mode adjacent to the C mode as the principal vibration lies closer to the high frequency side than the C mode by about 9%, and is sometimes more stressed than the C mode as the principal vibration.
Therefore, in the oscillator, oscillation in the B mode as unwanted vibration becomes a problem, or a frequency jump in which the oscillation frequency changes from that of the C mode as the principal vibration to that of the B mode as the unwanted vibration becomes a problem.
Various oscillators to solve this problem are proposed (See Patent Documents 1 and 2, for example). In an oscillator in Patent Document 1, as FIG. 8 shows a circuit configuration, in addition to an oscillating quartz vibrator X1 of a Colpitts oscillator, an AT-cut quartz vibrator X2 is inserted between a midpoint (division point) of a series circuit of divided capacitive components C1 and C2 and a connection point between an emitter of a transistor Q1 and a feedback resistance R3, and the series resonance frequency of the quartz vibrator X2 is set to substantially coincide with the oscillation frequency of the principal vibration (C mode). As a modified example thereof, as FIG. 9 shows a circuit configuration, in place of the quartz vibrator X2, a feedback circuit composed of a series circuit of an inductor L1 and a capacitive component C3 is provided. FIG. 10 shows a negative resistance characteristic in the configuration of FIG. 9.
In an oscillator in Patent Document 2, as FIG. 11 shows a circuit configuration, as a feedback circuit, divided capacitive components C3 and C4 are provided in parallel with a feedback resistance R4, and an inductor L1 and a variable reactance Z are provided from a connection point between the divided capacitive components C3 and C4. FIG. 12 shows a negative resistance characteristic in the configuration of FIG. 11.
(Patent Document 1)
Patent Publication No. 3268726
(Patent Document 2)
Patent Publication No. 3283493
An oscillator circuit in Patent Document 1 shown in FIG. 8 has a problem that it is expensive because it needs the B mode suppressing quartz vibrator X2 in addition to the oscillating quartz vibrator X1, that is, it needs two quartz vibrators.
In this regard, in an oscillator circuit shown in FIG. 9 (modified example of Patent Document 1), the B mode suppressing quartz vibrator X2 becomes unnecessary. However, when the series circuit of the capacitive component C3 and the inductor L1 is provided, it becomes difficult to obtain a characteristic equal to a frequency selection characteristic of the quartz vibrator X2 since its resonance characteristic is not so steep as that of the quartz vibrator. When the frequency of the principal vibration is high, the steepness of the resonance characteristic does not become a problem since the difference between a frequency corresponding to the C mode and a frequency corresponding to the B mode is large, but when the frequency of the principal vibration is low, the steepness of the resonance characteristic of the resonance circuit for frequency selection becomes a problem since the difference between the frequency corresponding to the C mode and the frequency corresponding to the B mode is small. In the case of this circuit configuration, as FIG. 10 shows the negative resistance characteristic, relative to a negative resistance value at the principal vibration (C mode) frequency, the same level of negative resistance value is shown even at the unwanted vibration (B mode) frequency, so that the suppression of the unwanted vibration cannot be expected.
An oscillator circuit in Patent Document 2 shown in FIG. 11 is designed so that by inserting the feedback circuit composed of the divided capacitive components C3 and C4 and the inductance (L1+Z) in a basic circuit of the Colpitts oscillator, the feedback circuit becomes a path at the frequency of the C mode and it is attenuated at the frequency of the B mode. As shown in FIG. 12, the negative resistance characteristic in this configuration exhibits an improvement in the negative resistance value at the frequency of the B mode as compared with that in FIG. 10.
However, in the example in FIG. 12, even at the frequency of the B mode, the negative resistance value is “−45Ω” which is a relatively high value, and hence there is a possibility that this negative resistance value causes the unwanted vibration of the B mode depending on the difference of the piezoelectric characteristic of the used quartz vibrator X1 and the mounting structure of the oscillator circuit.
Furthermore, in FIG. 11, the variable reactance Z can reduce the negative resistance value of the B mode by allowing oscillation frequency drift to be adjustable with respect to the occurrence of a constant error between circuit elements at the time of mounting, and besides by setting the peak of the negative resistance value to a frequency lower than that of the C mode by the adjustment by the reactance. However, in this frequency adjustment method, the negative resistance value of the C mode also reduces and it is difficult to increase the negative resistance value of the B mode to a positive value, so that the unwanted vibration cannot be surely suppressed.